Both natural processes and human activities can result in environmental toxins. The severity of toxicity varies with both expsoure to toxins and the susceptibility of particular genetic backgrounds. The evolutionary consequences of toxicity hinge on the interaction between genotypic variation and different exposure environments. The ideal model system for understanding these interactions is one where there is naturally occurring variation in susceptibility to naturally occurring toxins, such as heavy metal tolerance in Arabidopsis lyrata, the nearest relative of A. thaliana. In this systemexposureto heavy metals only occurs in a limited set of environments, and tolerance has somefitness cost, so that tolerance is not favored in all places. I will study genetic variation in heavy metal tolerance in A. lyrata through high-throughput genotyping of ecotypes from different soil types differing in heavy metal concentrations. Understanding natural variation in heavy metal tolerance, both in the presence and absence of exposure, is important to understanding evolutionary constraints on genes conferring tolerance, elucidating limits on tolerance, and for determining how tolerance can be improved in agricultural and environmental health contexts.